1. Field of the Invention
This invention relates to the handling of wafers, and more particularly to the handling of wafers in a wafer testing apparatus.
2. Background of the Related Art
The manufacture of integrated circuits (I.C.'s) begins with blank, unpatterned semiconductor wafers. These wafers undergo a number of sometimes critical process steps before being formed into the final I.C. form. A substandard wafer can affect the number of usable I.C.'s on a wafer (yield). It is therefore desirable to have a machine for testing wafers to ensure the wafers meet a customer's standards to maximize wafer yield.
The testing of wafers is often accomplished by an automated process, in which robots continuously handle and test the wafers. Robot testing and handling tends to be more efficient than manual testing and handling of wafers, since robots can be much faster and more precise than human operators. Also, robots tend to be less contaminating than humans when handling wafers.
Typical robot wafer handling systems move a wafer into testing position and move a probe assembly to engage the wafer surface and take measurements. Some robot systems also have the ability to remove a wafer from a wafer cassette, set it down on a testing surface, and place the wafer back into the wafer cassette after testing in a process known as cassette-to-cassette handling.
Prior art wafer handling systems vary in structure and method of handling wafers. In U.S. Pat. No. 4,204,155, M. Terry describes an automatic four-point probe mechanism that repeatedly lowers a four-point resistivity head onto a semiconductor wafer. In U.S. Pat. No. 4,755,746, Mallory et al. describes an automatic system for resistivity testing of semiconductor wafers, and includes a handling system whereby a wafer on a rotatable platform is lowered to a testing area. A probe assembly moves into place over the wafer and lowers so that the probe contacts the wafer to perform tests. In U.S. Pat. No. 4,907,931, Mallory et al. describes a semiconductor wafer handling apparatus which automatically moves wafers between wafer cassettes and a wafer test system. A shuttle arm is aligned with a wafer in a cassette and a spatula extends to remove the wafer from the cassette. The shuttle arm then rotates and moves the wafer to a testing area, where the arm lowers and retracts, leaving the wafer to be tested. The wafer is removed and placed into the cassette by the reverse operation.
While the prior art wafer handling and test systems have been successful in handling and testing wafers, they tend to exhibit some undesirable characteristics. For example, the prior art moves both the test probe and the wafer in the direction perpendicular to the wafer surface, where the test probe is moved to engage the probe with the surface of the wafer, and the wafer is moved to facilitate its loading and unloading. This duplication of movement increases costs of the wafer handling equipment due to the extra motors and controls that are required.
A further problem with the prior art wafer handling system is the complexity of the handling apparatus. The wafer-handling shuttle arm in the prior art requires several motors and must move along several axes to position a wafer correctly on a test area. Complex robots are expensive and require a great deal of maintenance to keep them operating correctly. Such costs and maintenance are undesirable in production environments.
What is needed is an apparatus and method that will handle and test a wafer efficiently and economically. Wafers would therefore be tested faster and with less production and maintenance costs, thereby increasing the amount of manufactured products and revenue.